Usually, as an evaporator for car cooler and the like, such a parallel flow type as shown in FIG. 1 has been generally used, in which deep recesses are formed at end portions of a metallic plate, two of which are communicated through shallow recess, putting an inner fin between them and joined together facing each other to form a unit pipe having upper and lower tanks 1 and 2 and a flat pipe 3, then a plurality of these unit pipes are arranged in parallel and fins 4 are inserted between flat pipes 3 respectively. Walls of adjacent tanks 1, 1 and 2, 2 are bored for mutual communication to form upper and lower tanks. 5 is an inlet pipe for cooling medium and 6 is an outlet pipe therefor.
Liquid cooling medium supplied from the inlet pipe 5 into the lower tank 2 flows through the flat pipes 3 into the upper tank 1, during which course the cooling medium absorbs heat of air flowing through the space between the flat pipes 3 and the fins 4.
In such a type of evaporator, however, it functions well when the flow rate of cooling medium fed from the inlet pipe 5 is sufficiently large, but as the flow rate thereof decreases, the quantity of the cooling medium flowing through each flat pipe 3 will become different between pipes located near the inlet pipe 5 and those far from it, and in extreme case the cooling medium does not flow utterly through remote pipes, but flows only through pipes adjacent to the inlet pipe to circulate within the evaporator, thus the hunting phenomenon becomes larger, resulting in remarkable trouble in the function of the evaporator.